1. Field of the Invention
This invention relates to the transmission and coherent detection of ultra wideband (UWB) waveforms in radar, sonar and communications systems, and more specifically to a method of transmitting and efficiently detecting self-conjugating waveforms using a general-purpose receiver.
2. Description of the Related Art
Research and development in the field of wideband radar systems has been undertaken for many years. The impetus for this development was rooted in the successful application of high power instrumentation radars for research in ballistic missile defense and satellite surveillance. Current wideband imaging radars provide considerable real-time discrimination and combat identification capability. Advanced signal processing methods have significantly improved the resolution of processed radar return signals, further improving the state-of-the-art in wideband radar technology. The ability to identify targets and accurately estimate their size and shape is critical to many applications.
World War II era radar transmitted simple pulsed signals and detected only the power of a pulse returned from targets. Modern radars may transmit much more complex waveforms. To extract all possible information contained in the returning signals, many modern radars use coherent detection to extract both amplitude and phase information. The additional information can be used to, for example, measure the closing velocity of the target or form terrain images in synthetic aperture radars (SARs).
Typical relatively wideband radar systems transmit a burst of radio frequency energy which consists of many cycles at a center frequency. Such a wideband pulse is characterized by Δf/f<<1 where f is the center frequency and Δf is the pulse bandwidth or 1/(pulse duration). For example, a 30 ns pulse at 10 Ghz will transmit 300 cycles and Δf/f=1/300. The return signal resembles a sine or cosine wave within the pulse envelope. To capture all possible received energy, the receiver coherently detects both in-phase and quadrature signals by mixing the received signal with cosine and sinusoidal local oscillators. The amplitude information is extracted by squaring and adding the in-phase and quadrature detected signals to recover the target's response. Phase information can be derived mathematically by calculating the arctangent of the ratio of the in-phase to quadrature signal amplitudes. Continuing with the example, flying at the speed of light the 30 ns, 10 Ghz pulse has a pulse length of 9 meters. This signal will provide a range uncertainty to the target of approximately 9 meters in the round trip distance or 4.5 meters in the one-way range to the target.
To achieve fine range resolution, some radar systems utilize coded waveforms with large time-bandwidth products. Wideband “chirp” waveforms are commonly used in practice due to their ease of generation and processing in the radar receiver. By mixing the radar return signals with a replica of the transmitted signal, a baseband signal is produced with frequency components that are proportional to the relative range between scattering centers on the target. The ALCOR C-band radar utilizes a wideband chirp waveform with a bandwidth of 512 MHz providing ALCOR with a range resolution capability of about 53 cm. Kwajalein's millimeter-wave radar (MMW) can operate at the Ka- and W-bands and is capable of a transmission bandwidth of 2000 MHz, providing an impressive 14 cm range resolution capability (U.S. Pat. No. 5,945,940).
Another approach is to transmit simple ultra widband (UWB) pulses which consist of a single-cycle or a few-cycle waveform where the bandwidth is comparable to the center frequencey, i.e., Δf/f≈1. Fourier theory dictates that the achievable resolution is inversely proportional to the total waveform length. This means that range resolution improves as radar bandwidth increases. For example, a 1 ns pulse at a center frequency of 1 Ghz would have approximately one cycle of oscillation and would provide a one-way range resolution of 0.15 meters or about 6 inches. With a 10 GHz center frequency, a single-cycle waveform could provide resolution 10-times finer or 0.6 inches. UWB pulses can be generated using very high speed switching, harmonic oscillators (see U.S. Pat. Nos. 5,146,616 and 5,239,309) or by chopping sine or cosine signals.
The problem with UWB pulses is how to detect them without expensive special purpose hardware and still recover the maximum energy and range resolution. The coherent detector described above is only effective for detecting relatively narrow band signals which have many cycles and are thus are approximately sinusoidal. The power spectrum of a returned UWB pulse is very broad and thus would only appear as a short transient in the noise of the detector. Typical UWB systems utilize some type of special purpose “matched” reciever such as a correlator, which can be be very expensive. Matched recievers must be designed for a specific UWB waveform. Because the correlations are about twice as long as the original waveforms, the range resolution is degraded by at least a factor of two, negating some of the advantage of UWB waveforms. In addition, because the correlation operation is a non-linear function interference problems may arise when several overlapping pulses are simultaneously returned to the correlator.
To avoid the complexity and expense of true UWB systems, Cuomo in U.S. Pat. No. 5,945,940 proposes a radar system that coherently combines signals from independent upper- and lower-sub-band radars, mutually coheres the sub-band radar signals, and performs model fitting and parameter estimation to obtain ultra-wideband data signatures from a target. Signal processing models are used to compensate for potential lack of mutual coherence between the various sub-bands. An ultra-wideband signal model is fitted to the sparse sub-band measurements to accurately characterize ultra-wideband target scattering and provide for meaningful interpolations or extrapolations outside of the measurement sub-hands.
Before UWB radar will become widely accepted, a general purpose approach for transmitting and detecting simple UWB pulses is needed that preserves both range resolution and recovers all possible energy.